Devolatilization

Abstract

The efficiency of falling strand devolatilizers is related to a number of factors incluidng the foaming of the polymer melt and the time the melt in both foam and strand form is held in the devolatilizer. The level of residual material including monomers, dimers, trimers, and diluents, in a polymer melt may be reduced by passing the melt sequentially through a preheater operated at relatively low pressure then through a horizontal melt distributor at the top of a devolatilizer chamber.

What is claimed is: 
     
       1. A horizontal polymer melt distributor used in the upper end of a reduced pressure devolatilizer said distributor comprising: an inlet means and a die assembly which cooperates with and receives polymer melt from a preheater, said die assembly comprising two oppositely facing flange plates connected about a central die plate said flange plates having essentially equisized central openings therethrough having an inward taper towards said die plate, said die plate bearing a number of holes therethrough connecting the central openings through said flange plates, the total cross sectional area of said holes being less than the minimum cross sectional area of the central openings through said flange plates, a heated enclosed first section having a cross sectional area greater than the minimum cross sectional area of said inlet, said first section opening into at least one subsequent at least partially heated venting and distributing sections which bear on their upwardly facing surface at least one aperture and on their downwardly facing surface strand forming holes permitting the exit of said polymer melt at a rate substantially equal to the rate polymer melt enters said distributor, the total cross sectional area of said one or more subsequent sections being at least equal the cross sectional area of said first section, said first and subsequent sections being adapted so that a substantially linear flow of polymer melt is maintained through out said distributor, with no stagnant areas.   
     
     
       2. The distributor according to claim 1 wherein the holes through said die plate are tapered. 
     
     
       3. The distributor according to claim 2 wherein the shape of said distributor is selected from the group consisting of an annulus, a serpentine, a toroid and a trident. 
     
     
       4. A distributor according to claim 3 wherein said strand forming holes are sized so that the strands of polymer melt descending to the bottom of the devolatilizer are essentially equisized continuous strands. 
     
     
       5. The distributor according to claim 4 wherein said at least one subsequent at least partially heated venting and distributing sections further contain a series of over flow holes located between the at least one aperture and the strand forming holes. 
     
     
       6. The distributor according to claim 5 in the shape of a trident. 
     
     
       7. The distributor according to claim 6 wherein the first section comprises the central arm of the trident and is a barrel heated by a jacket or coil heater. 
     
     
       8. The distributor according to claim 7 wherein the at least one subsequent sections are the outer tines of the trident and comprise closed end tubes having a total cross sectional area from 1.2 to 1.7 times the cross sectional area of the first section which are at least partially heated with bayonet heaters. 
     
     
       9. The distributor according to claim 8 wherein said at least one aperture in said at least one subsequent sections are in the upward surfact of the tines facing the inner surface of the devolatilizer and extend at least 50% of the length of said tines. 
     
     
       10. A process for devolatilizing a polymer melt in a reduced pressure devolatilizer comprising pumping said melt through a preheater at a temperature of from 230° to 250° C. and a pressure from 10 to 30 psig so that the vapor pressure of residual monomer, dimers, trimers and diluent in said melt is greater than the pressure within said preheater; flowing said melt through a horizontal inlet and die plate, which die plate has a number of holes therethrough, the number and diameter of said holes being less than the cross section area of said inlet to maintain a pressure of 10 to 30 psig in said preheater; horizontally flowing said polymer melt into an enclosed first expansion zone in said devolatilizer, said enclosed first expansion zone having a cross-section area greater than said inlet, and at pressure from 5 to 25 mm of Hg and heating said melt to a temperature of from 230° to 250° C. to cause said polymer melt to foam, linearly flowing said melt into at least one subsequent zone having a cross sectional area at least equal to the cross sectional area of said enclosed first expansion zone and having apertures open to the interior of said devolatilizer at a pressure of 5 to 25 mm of Hg to expose the polymer melt foam to the interior of the devolatilizer while heating at least part of the polymer foam to a temperature from 230° to 250° C. and permitting the foam to flow through strand forming holes in the bottom of said subsequent zone at a rate substantially equal to the rate polymer melt flows into said first expansion zone to form a plurality of strands of polymer foam which descend to the bottom of the devolatilizer and pumping said polymer melt from said devolatilizer.